1. Field of the Invention
The control of vibrations in composite structures is an important area of research in aerospace, automotive and other industries. For example, spacecraft vibrations initiated by altitude adjusting thrusters, motors and thermally induced stresses inhibit accurate aiming of antennas and other equipment carried by the craft. Such vibrations can cause severe damage to the craft and its associated equipment. Fatigue failure of structural components can occur at stresses well below static load limits.
Composite materials have been used to construct a wide variety of structural elements, including tubes, enclosures, beams, plates and irregular shapes. Objects as diverse as rocket motor housings and sporting goods, notably skis, archery arrows, vaulting poles and tennis rackets have been structured from composite materials. While composite constructions have offered many significant advantages, such as excellent strength and stiffness properties, together with light weight, the poor vibration damping properties of such constructions have been of concern.
The invention relates to composite material structures having increased damping with little or no sacrifice of structural stiffness or strength.
The invention also relates to the methods and apparatus for manufacturing the aforementioned composite material structures.
Another aspect of the invention is directed toward the fabrication of a wavy fiber pregreg (fibers preimpregnated with epoxy resin). Such prepregs not only have an aesthetic appeal but also may be fabricated with selected variable volume fractions to accommodate a variety of applications.
2. Description of Related Art
The following terms used herein will be understood to have their ordinary dictionary meaning as follows:
Fiber: a thread or a structure or object resembling a thread. PA1 Matrix: material in which something is enclosed or embedded. PA1 Viscoelastic: having appreciable and conjoint viscous and elastic properties. PA1 Lamina(e): a thin plate . . . : LAYER PA1 Composite: made up of distinct parts. PA1 a first pair of rollers including a first and second roller, PA1 at least a third roller, PA1 the fiber tow pressed between the first and second rollers and contacting the third roller for movement along a first direction due to rotation of the first, second and third rollers, a first drive mechanism for rotating the first, second and third rollers, PA1 a second drive mechanism connected to at least one of the first pair of rollers and the third roller, the second drive mechanism responsive to control signals for displacing the at least one of the first pair of rollers and the third roller axially along the rotation axis of the at least one of the first pair of rollers and the third roller, and PA1 an electronic controller connected to the second drive mechanism for generating the control signals to control the amount of axial displacement of the at least one of the first pair of rollers and the third roller. PA1 a fourth roller positioned adjacent the third roller to form a second pair of rollers, PA1 the fiber tow passing between the third and fourth rollers, PA1 at least a first and second support roller and a first and second sheet of matrix material, PA1 the first support roller supporting the first sheet of matrix material and the second support roller supporting the second sheet of matrix material, and PA1 the first support roller positioned for feeding the matrix material of the first sheet between one of the third and fourth rollers and the fiber tow, and the second support roller positioned for passing the matrix material of the second sheet between the other of the third and fourth rollers and the fiber tow.
a slender and greatly elongated natural or synthetic filament. (This definition includes metal fibers).
It may be further helpful to note the definitions of G.sup.0 and G.sup.1 geometric continuity. "If two curve segments join together, the curve has G.sup.0 geometric continuity. If the directions (but not necessarily the magnitudes) of the two segments' tangent vectors are equal at a join point, the curve has G.sup.1 geometric continuity. G.sup.1 continuity means that the geometric slopes of the segments are equal at the join point." Foley et al. Computer Graphics Principles and Practice, Addison-Wesley, 1996, p. 480.
The article "Understanding Vibration Measurements," George F. Lang, Sound and Vibration, March, 1976, p. 26, presents a generally accepted mathematical treatment of the vibration of mechanical structures under environmental loading. This treatment applies generally to composite structures, and is incorporated by reference for purposes of this disclosure for its explanation of the amplification factor Q and its relationship to the viscous damping factor .zeta.. The "loss ratio" referred to in this disclosure is twice the viscous damping factor as defined by Lang.
Conventional methods used to control the often destructive levels of vibration take many forms, from simple passive treatments to extensive redesign of structures. One of the simplest and often most effective passive damping treatments involves the use of thermo-visco-elastic (TVE) materials. TVE materials such as 3M's Scotchdamp series (ISD-112 is one example), exhibit dissipative qualities which make them useful in a number of passive damping treatments. Some of the first uses of TVE materials to increase structural damping involved the use of surface patches of aluminum foil and viscoelastic adhesives. These conventional approaches to surface damping treatments are called constrained or embedded-layer damping, and produce modest gains in damping over undamped structures.
One of the more common passive damping methods, "constrained layer damping" or CLD is discussed in the article "Damping of Flexural Waves by a Constrained Viscoelastic Layer," Kerwin, Journal of the Acoustical Society of America, 1959, Vol. 31, Issue 7, pp. 952-962. According to Kerwin, CLD is achieved by bonding a thin layer of metal sheet, usually aluminum, to an existing structure with a viscoelastic adhesive. According to this technique, damping material, typically a viscoelastic material, is applied to the surface of a composite structure, such as an airplane wing. The damping material is sandwiched between the composite surface and a rigid layer, such as a thin aluminum sheet. This approach has generally been remedial in character and is accomplished at the sacrifice of other considerations, such as weight, aesthetics and ideal surface configuration. Shear strains are developed in the viscoelastic material when the original structure bends or extends. Damping occurs when the deformation of the viscoelastic adhesive creates internal heat in the viscoelastic material and dissipates energy.
Compared to an undamped structure, this approach is modestly successful but its effectiveness decreases markedly as the ratio of the thickness of the base structure to the thickness of the viscoelastic material increases. Thus, surface treatments alone cannot provide significant levels of damping to structural members where greater strength and stiffness are important. In the article "Use of Strain Energy Based Finite Element Techniques in the Analysis of Various Aspects of Damping of Composite Materials and Structures," Hwang, et al., Journal of Composite Materials, 1992, Vol. 26, Issue 17, pp. 2585-2605, this problem was reported, and it showed that the advantage of aluminum foil viscoelastic constrained layer damping was eclipsed by the inherent damping in conventional composites when the required thickness of the structure exceeded about three tenths of an inch. The authors determined that a .+-.45.degree. graphite/epoxy composite provided approximately uniform damping of about 1.5% in thick sections.
It is known that laminated beams composed of alternating layers of elastic and viscoelastic materials can dissipate vibratory energy while maintaining a degree of structural integrity. The article "Composite Damping of Vibrating Sandwich Beams," DiTaranto, et al, Jour. of Engineering for Industry, Nov., 1967, p. 633, presents a theoretical description of such structures.
The feasibility of co-curing embedded layers of damping materials in a composite structure has been demonstrated. See, for example, Rotz, Olcott, Barrett, "Co-cured Damping Layers in Composite Structures," Proceedings 23rd International SAMPE Technical Conference, Vol. 23, pp. 373-387, 1991. Vibration control can thus be designed into a structure prior to its actual construction. Composite tubes have been constructed from a pair of concentric composite stiffness layers, the annular space between them being occupied by a damping layer. It is known that when fiber-reinforced materials are loaded along any axis not parallel or perpendicular to the fibers, shear deformations are induced. A tube with plies oriented "off-axis" (with respect to the central axis of the tube) will twist when loaded axially. With the plies of the two stiffness layers oriented at opposite but similar angles with respect to the tube axis, intense shear deformation is induced in the damping layer when the tube is loaded axially. The stiffness of the tube remains high because the load passes only through the stiffness layers, and the damping layer adds little weight to the structure. Unfortunately, the most significant shear displacements occur at the free ends of the tube. Constraining the rotational deformations of the stiffness plies at either end of the tube eliminates any shear deformations in the damping layer at that end, thereby reducing the damping effect of the system. Complicated end fixtures are thus required to allow the requisite free end displacements while still transmitting axial loads.
Stress-coupled co-cured composite viscoelastic structures are formed when layers of uncured fiber composites and TVE materials are alternately stacked together and co-cured in an oven. These structures provide impressive levels of damping and can be categorized by the fiber orientation methods used to induce damping in the TVE material.
The article "A Design for Improving the Structural Damping Properties of Axial Members," Barrett, Proceedings of Damping, 1989, Vol. HBC-1-18, proposed designs using conventional angled-ply (.+-..theta.) composite lay-ups of straight fiber pre-preg materials (e.g., fabric layers preimpregnated with resin material). Barrett used the inherent shear coupling properties of composite materials to design damped composite tubular components which would induce significant damping in the structure. Fiber-reinforced composites will shear as the fibers attempt to align themselves with the applied loads when these loads are not parallel or perpendicular to the fiber. Because of this behavior, a plate constructed with a positive .theta. fiber layer, a viscoelastic material layer, and a negative .theta. fiber layer will generate large shear strains (.gamma.) in the TVE material when an axial load is applied to the tube. Barrett's research showed that maximum shearing was experienced at the ends of the tubes. However, connections at the tube ends eliminated much of the damping effect, rendering the design impractical for many applications.
U.S. patent application Ser. No. 07/780,923 to Olcott et al., filed Oct. 22, 1991, proposes "stress coupled damping" (SCD) which also uses conventional angled ply (.+-..theta.) composite lay-ups of straight fibers, but abruptly changes the fiber orientation several times throughout the structure. According to Olcott et al., each composite layer is comprised of multiple segments of pre-preg composite material. The patent teaches the use of adjacent segments, having fibers therein which cumulatively form a chevron pattern. The composite layer may include several pre-preg plies in which the segments are staggered or overlapped to strengthen the joint. This creates a region in the composite layer that exhibits quasi-isotropic properties.
By selecting the fiber angle, thickness, and segment lengths, significant shearing in the viscoelastic layers was observed over the entire structure, not just at the ends as in Barrett's design. As a result, the structure retained high stiffness and exhibited improved damping even when the ends were clamped. The following publications, incorporated herein by reverence, are cited for further details on this subject.
1. Olcott, D. D., "Improved Damping in Composite Structures Through Stress Coupling, Co-Cured Damping Layers, and Segmented Stiffness Layers," Brigham Young University, Ph.D. Thesis, August 1992.
2. Trego, A., Eastman, P. F., Pratt, W. F., and Jensen, C. G., "Reduced Boring Bar Vibrations Using Damped Composite Structures," Proceedings of the 1995 Design Engineering Technical Conferences, Vol. 3, Part A, pp. 305-311.
3. Trego, A. and Eastman, P. F., "Optimization of Passively Damped Composite Structures," International Journal of Modelling and Simulation, Vol. 17, No, 4, 1997;
4. Trego, A., Olcott, D. D., and Eastman, P. F., "Improved Axial Damping of Mechanical Elements Through the Use of Multiple Layered, Stress Coupled, Co-Cured Damped Fiber Reinforced Composites", Journal of Advanced Materials, January, 1977, p.28.
The text Vibration Damping of Structural Elements, by C. T. Sun and Y. P. Lu, Prentice Hall PTR, 1995, presents a discussion of fiber reinforced damping in composite materials using viscoelastic materials for damping, and is incorporated herein by reference.
Although damping was improved, such structures manufactured by conventional lay-ups of straight fiber segments were time consuming to make, error prone, and could not be readily automated. The use of composite struts to dampen vibrations has been proposed in U.S. Pat. No. 5,203,435 to Dolgin. According to Dolgin, plies with opposing chevron patterns of fibers would convert longitudinal vibrational stresses into shear stresses in an intermediate viscoelastic layer which would then dissipate the vibrational energy. One specific structure proposed is a tube with an inner ply oriented at 0.degree., a second layer oriented at+45.degree., a central viscoelastic layer, a subsequent layer oriented at -45.degree., and an outer layer oriented at 90.degree.. Dolgin also illustrates a sinewave pattern. The disclosure of the Dolgin patent is incorporated herein by reference.
The materials, methods and applications of composites manufacturing are set forth in detail in the literature. An exemplary source for information of this kind is the textbook "FUNDAMENTALS OF COMPOSITES MANUFACTURING" by A. Brent Strong, published by The Society of Manufacturing Engineers, Dearborn, Mich., 1989. The disclosure of this textbook is incorporated herein by reference for its explanation of the art of composites manufacturing as it is currently practiced. Similarly, the textbook "VISCOELASTIC PROPERTIES OF POLYMERS," John D. Ferry, Wiley, N.Y., (3rd Ed., 1980) presents a thorough summary of the nature, behavior and identity of typical materials which may be selected by composites technicians for use as damping materials. A good discussion of viscoelasticity as it applies to materials of interest to this disclosure is presented in the publication "Linear Viscoelasticity, Driscoll, et al., available from the Structural Products Department of 3M Company, St. Paul, Minn.
There remains a need for a composite structure capable of diverse configuration with improved damping characteristics and which avoids the limitations of the structural approaches heretofore suggested for use with composites materials.